The clinical circumstances of the preterm and sick infant are quite complex from a metabolic and nutritional viewpoint. The calculation of protein catabolism and synthesis in infants with a wide variety of disease states could be used as a guideline to help recognize the infants who are at a particular nutritional risk and identify those who are becoming increasingly catabolic. Information gained from this study will provide a more accurate estimate of protein requirements by neonates and potentially maximize their nutritional status.